The present invention relates to a process for the production of polyolefins, in particular polyethylene or polypropylene. In particular, the present invention relates to the production of a polyethylene having a multimodal molecular weight distribution, for example a bimodal molecular weight distribution.
It is known to produce polyethylene in liquid phase loop reactors in which ethylene monomer, and optionally an alpha-olefinic comonomer typically having from 3 to 10 carbon atoms, are circulated under pressure around a loop reactor by a circulation pump. The ethylene monomer and comonomer when present are present in a liquid diluent, such as an alkane, for example isobutane. Hydrogen may also be added to the reactor. A catalyst is also fed to the loop reactor. The catalyst for producing polyethylene may typically comprise a chromium-based catalyst, a Ziegler-Natta catalyst or a metallocene catalyst. The reactants in the diluent and the catalyst are circulated at an elevated polymerisation temperature around the loop reactor thereby producing polyethylene homopolymer or copolymer depending on whether or not a comonomer is present. Either periodically or continuously, part of the reaction mixture, including the polyethylene product suspended as slurry particles in the diluent, together with unreacted ethylene and comonomer, is removed from the loop reactor.
The reaction mixture when removed from the loop reactor may be processed to remove the polyethylene product from the diluent and the unreacted reactants, with the diluent and unreacted reactants typically being recycled back into the loop reactor.
Alternatively, the reaction mixture may be fed to a second loop reactor serially connected to the first loop reactor where a second polyethylene fraction may be produced. Typically, when two reactors in series are employed in this manner, the resultant polyethylene product, which comprises a first polyethylene fraction produced in the first reactor and a second polyethylene fraction produced in the second reactor, has a bimodal molecular weight distribution.
It is known in the art to operate a loop reactor under conditions of high temperature and pressure such that the diluent is present under supercritical conditions. Thus the diluent is at a pressure greater than the critical pressure Pc and at a temperature greater than the critical temperature Tc. Under these conditions, there is no thermodynamic transition between the gas phase and the liquid phase and the homogeneous supercritical fluid has the properties of a dense gas and a low density liquid.
For example, WO-A-92/12181 discloses a method for homo- or copolymerising ethene in the presence of a Ziegler-Natta catalyst in a loop reactor under supercritical conditions. The diluent which is in the supercritical state is propane. It is disclosed that the use of a propane phase at a supercritical state provides some advantages, namely that the hydrogen content of the reactor may be adjusted within a wide range and no pressure-shock effects occur which would otherwise tend to damage the circulation pump for the diluent, as a result of the high compressibility of the supercritical fluid. This specification makes clear that propane should be used as a diluent rather than for example isobutane, because the use of propane enables more polymer types to be prepared in the reactor and also the solubility of polyethylene is lower in propane than in isobutane. The specification also discloses that since the boiling point of propane is low, the hydrocarbons may readily be separated from the polymer particles after the polymerisation. The specification discloses that two loop reactors in series may be employed for making ethylene polymers and/or copolymers having a wide or bimodal molecular weight distribution.
EP-B-0517868 also discloses a multi-stage process for producing polyethylene which employs supercritical conditions. It is disclosed that the inert hydrocarbon medium which is employed under supercritical conditions is propane. It is also disclosed that the polyethylene may have a bimodal molecular weight distribution.
WO-A-96/18662 discloses a process for preparing polyethylene which may have a multimodal molecular weight distribution by using supercritical conditions. Again, it is disclosed to be advantageous to use propane as the inert hydrocarbon medium under supercritical conditions.
WO-A-96/34895 discloses a process for manufacturing LLDPE polymers again using propane as a reaction medium under supercritical conditions. The LLDPE polymers are manufactured using a metallocene catalyst. It is disclosed that the excellent polymer morphology of the products produced with the metallocene catalysts together with the low polymer solubility into the diluent and relatively low diluent density, especially in the supercritical conditions, result in very good settling properties of the polymer and thus efficient reactor operation, (i.e. diluent flow into the reactor can be minimised). However, there is no disclosure of any specific reactor structure indicating how the operation of the reactor may be made more efficient.
WO-A-97/13790 discloses a process for making propylene homo- or copolymers in a loop reactor under supercritical conditions. It is disclosed that a polypropylene having a bimodal molecular weight distribution may be employed using two reactors in series.
While the above-specified patent specifications relating to supercritical conditions for the diluent provide advantages of higher hydrogen solubility in the diluent and easier hydrogen flashing if the reaction has been continued in the second reactor, combined with reduced swelling of the polymer in the supercritical diluent and the absence of pressure shocks as a result of the high compressibility of the supercritical diluent, nevertheless, the use of propane as a diluent tends to require the use of comonomers having low carbon numbers, for example butene which militates against the use of high carbon comonomers, for example hexene which would assist in the production of polymers having better properties than if butene were used. Moreover, the use of propane as a diluent tends to require a relatively high pressure to be employed above the critical pressure Pc for propane. Moreover, the supercritical processes referred to hereinabove do not permit a particularly high comonomer concentration to be employed in the reactor, particularly for comonomer with high carbon number, e.g. hexene.
In the manufacture of polyolefins having a bimodal molecular weight distribution under supercritical conditions employed in serially connected reactors, the above-identified specifications suffer from the disadvantage that there is no specific disclosure as to how the reaction medium is transferred from the first reactor to the second reactor.
U.S. Pat. No. 4,754,007 discloses a process for copolymerising ethylene to form LLDPE copolymers in which liquid propane is used as a diluent in a slurry process. It is disclosed that the use of propane diluent provides more economical production of copolymers having more desirable physical properties than slurry processes using isobutane, hexane or other liquid diluents. There is no disclosure of the diluent being under supercritical conditions.
EP-A-0649860 discloses a process for the copolymerisation of ethylene in two liquid full loop reactors in series in which the average molecular weight is regulated. A comonomer is introduced into the first reactor and high and low average molecular weight polymers are produced respectively in the first and second reactors. One or more settling legs is provided for the first reactor in order to transfer the high average molecular weight polymer from the first reactor to the second reactor. The reaction is carried out in a diluent, for example isobutane, in a slurry process. This process suffers from the disadvantage that although the use of settling legs for concentrating the fluff between the first and second reactors allows preferential polymerisation of comonomer in the high molecular weight fraction nevertheless the comonomer amounts in the first and second reactors are rather close because the reactors do not operate substantially independently. It would be desirable to achieve lower C6/C2 ratios in the second reactor, thereby yielding improved properties for the resultant polyolefin resin.
U.S. Pat. No. 4,740,550 discloses a multi-stage, continuous polymerisation process for the preparation of propylene/ethylene impact copolymers comprising the use of a re-circulating pipe-loop reactor for homopolymerising propylene, a cyclone separator for removing fines, a gas-phase fluidised bed reactor for additional propylene homopolymerisation, and a gas-phase fluidised bed reactor for propylene/ethylene copolymerisation. The essence of the disclosure is that since the first reactor is operated under slurry conditions and the second reactor is operated under gas phase conditions, a hydrocyclone separator is employed to separate the fine particles from the coarse fluff particles that are fed to the gas phase reactor. The slurry phase reactor operates with a liquid diluent and the fine particles are recycled back to the first slurry phase reactor. This process requires the reactors to operate in the liquid and gas phases, and the use of a hydrocyclone which is inconvenient.
EP-A-0905153 discloses a process for producing high density polyethylene in the presence of a Ziegler-Natta catalyst system in two liquid full loop reactors in series. The reactors are both operated with a liquid diluent, for example isobutane. In a first reactor there is substantially homopolymerisation, optionally with a minor degree of copolymerisation, and hydrogen is introduced into the first reactor to achieve the required homopolymerisation. Copolymerisation is carried out in the second reactor. In order to reduce or prevent hydrogen from entering the second reactor, a hydrogenation catalyst is introduced into the reactants downstream of the first reactor. This process requires the use of an additional hydrogenation catalyst.
The present invention aims at least partially to overcome these problems of the prior art.
Accordingly, the present invention provides a process for producing polyolefins having a bimodal molecular weight distribution, the process comprising producing a first polyolefin fraction in the presence of a catalyst in a first loop reactor, and producing a second polyolefin fraction in the presence of the catalyst in a second loop reactor which is serially connected to and downstream of the first loop reactor, the first and second polyolefin fractions being blended in the second loop reactor to form a polyolefin having a bimodal molecular weight distribution, at least the first loop reactor containing a diluent under supercritical conditions which is circulated around the loop of the reactor, and wherein at least the first loop reactor is provided with a fluff concentrating device communicating with the loop and in which polyolefin fluff of the first fraction is concentrated in the supercritical diluent, and polyolefin fluff of the first polyolefin fraction is transferred together with an amount of supercritical diluent from the fluff concentrating device of the first loop reactor into the second loop reactor.
The polyolefin may comprise polyethylene or polypropylene. When producing polyethylene, the diluent typically comprises at least one C1 to C4 alkane. When producing polypropylene, the diluent typically comprises propylene.
Preferably there is provided a process wherein the fluff concentrating device is selected from one or a combination of a downwardly depending settling leg, a cyclone or hydrocyclone and a centrifuge.
More preferably the fluff concentrating device includes a valve for permitting an amount of the polyolefin fluff together with an amount of the supercritical diluent periodically to be removed from the fluff concentrating device.
In one preferred aspect there is provided a process wherein the diluent in the second loop reactor is operated under supercritical conditions and the second loop reactor is provided with a respective fluff concentrating device.
More preferably there is provided a process further comprising recycling back into the first and second loop reactors any diluent removed from the fluff concentrating device of the second loop reactor on removal of polyolefin fluff therefrom.
In another preferred aspect there is provided a process wherein the diluent in the second loop reactor is operated under liquid conditions and the second loop reactor is provided with a respective fluff concentrating device.
The present invention further comprises the use, in a pair of serially connected loop reactors for polymerising an olefin in the presence of a catalyst to produce a polyolefin having a bimodal molecular weight distribution and including a first polyolefin fraction produced in a first loop reactor and a second polyolefin fraction produced in a second loop reactor downstream of the first loop reactor, of a diluent under supercritical conditions in at least the first loop reactor for increasing the settling of polyolefin fluff in a respective fluff concentrating device of at least the first loop.
The present invention yet further provides the use, in a pair of serially connected loop reactors for polymerising ethylene in the presence of a catalyst to produce polyethylene having a bimodal molecular weight distribution and including a first polyethylene fraction comprising polyethylene copolymer produced in a first loop reactor and a second polyethylene fraction comprising polyethylene homopolymer produced in a second loop reactor downstream of the first loop reactor, of a diluent under supercritical conditions in at least the first loop reactor for reducing the amount of comonomer in solution in the diluent transferred from the first loop reactor to the second loop reactor.
The present invention also provides the use in a pair of serially connected loop reactors for polymerising ethylene in the presence of a catalyst to produce polyethylene having a bimodal molecular weight distribution and including a first polyethylene fraction comprising polyethylene homopolymer produced in a first loop reactor and a second polyethylene fraction comprising polyethylene copolymer produced in a second loop reactor downstream of the first loop reactor, of a diluent under supercritical conditions in at least the first loop reactor for reducing the amount of hydrogen in solution in the diluent transferred from the first loop reactor to the second loop reactor.
The present invention still further provides the use, in a pair of serially connected loop reactors for polymerising an olefin in the presence of a catalyst to produce a polyolefin having a bimodal molecular weight distribution and including a first polyolefin fraction produced in a first loop reactor and a second polyolefin fraction produced in a second loop reactor downstream of the first loop reactor, of a diluent under supercritical conditions in the first and second loop reactors for reducing the amount of diluent to be recycled back into the loop reactors following removal of a mixture of polyolefin fluff and diluent from a fluff concentrating device of the second loop reactor.
Through this process the second reactor is made more independent from the first one.